Applied Physics Letters, 31 March 2008
Appl. Phys. Lett. 92, 133305 (2008) (3 pages)
©2008 American Institute of Physics. All rights reserved. Rightslink - Permissions for ReusePermissions for ReuseAbout Rightslink
Up: Issue Table of Contents
Go to: Previous Article | Next Article
Other formats: HTML (smaller files) | PDF ( kB)

The effect of molecule-molecule and molecule-substrate interaction in the formation of Pt-octaethyl porphyrin self-assembled monolayers

Nuri Oncel and Steven L. Bernasek(a)

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA

(Received: 6 January 2008; accepted: 10 March 2008; published online: 1 April 2008)

The adsorption of Pt-octaethyl porphyrin (Pt-OEP) molecules on highly ordered pyrolytic graphite (HOPG) and on 5-(octadecyloxy) isophthalic acid covered HOPG was studied with scanning tunneling microscopy at the solid-liquid interface. Pt-octaethyl porphyrin molecules lie flat on both surfaces and form a hexagonal lattice with an internal angle of about 60° and a lattice spacing of approximately 1.2  nm. The similar overlayer structure observed on the dramatically different substrates suggests that molecular packing in the Pt-OEP layer controls the structure. ©2008 American Institute of Physics

Contents

The paper of Aviram and Ratner1 suggesting the possibility of using individual molecules as a rectifying component in an integrated circuit has initiated many studies on the physical properties of individual molecules.2,3,4 The goal of manufacturing devices from individual molecules and employing these devices in everyday applications has not yet been reached. Thin films of molecules with certain physical and chemical properties have been implemented in various electronic and optoelectronic devices such as electroluminescent devices,5 sensors,6 diodes,7 and photovoltaic cells.8 The performance of these devices strongly depends on the quality of the molecular films and the interfaces between them. High quality molecular films and interfaces can be realized with the help of self-assembly. Molecular self-assembly is due to the mutual interactions between the molecules ranging from weak and nondirectional van der Waals bonds to strong and directional hydrogen bonds.9,10,11 Moreover, the electronic and the topographic properties of the substrate and its interaction with the molecules determine the adsorption strength and the adsorption geometry of the molecules, which eventually play a crucial role in the molecular film and the interface quality and its electronic properties. A significant leap in the understanding of the electronic and the topographic properties of the individual molecules12 as well as molecular films13 has occurred with the invention of the scanning tunneling microscope (STM), and it is the method used in the study described here.14

Metal-porphyrins are one of the most closely examined organometallic complexes due to their importance in many biological processes15 and possible applications in photoelectric devices.16 The Pt-octaethyl porphyrin (Pt-OEP) molecule is a quite important molecule due to its implementations in phosphorescent organic light emitting diodes.17,18 We report here a STM study of the adsorption of Pt-OEP on the highly oriented pyrolytic graphite (HOPG) substrate at the solid-liquid interface. We have also studied the adsorption of Pt-OEP on the HOPG surface covered with a 5-(octadecyloxy) isophthalic acid (5-OIA) monolayer. The HOPG substrate was purchased from Nanoscience Instruments. Solvent 1-phenyloctane (98%), Pt-OEP (98%), and 5-OIA (98%) were purchased from Aldrich. Pt-OEP and 5-OIA were used without further purification. STM tips (Pt0.8Ir0.2, 0.25  mm diameter) were purchased from Nanoscience Instruments. Experiments were performed at the solid-liquid interface and under ambient conditions.

A high resolution STM image of Pt-OEP molecules adsorbed from 1-phenyloctane solution (solution A: 0.002  g of Pt-OEP in 5  ml 1-phenyloctane) on the freshly cleaved HOPG surface is shown in Fig. 1(a). Pt-OEP molecules appear as protrusions in the image. Due to tunneling through the half filled dz2 orbital of Pt, Pt-OEP does not exhibit a depression in the center of the molecule.19 Topographical features of Pt-OEP molecules do not show significant variations in a voltage interval ranging from 0.6  to  1  V, with unstable tunneling conditions outside of this voltage range. The Pt-OEP self-assembled monolayer (SAM) forms a hexagonal lattice with an internal angle of about 60°. The lattice spacing is extracted from the line scans of the STM images and is found to be smaller (a=b~=1.2  nm) than the spacing observed for Ni-OEP (a=1.55±0.03  nm and b=1.47±0.06  nm) adsorbed on HOPG from a benzene solution.20 Previous studies on the self-assembly of molecules at the solid-liquid interface show that the solvent may affect the adsorption geometry.21,22 In order to investigate this possibility, a solution of 0.002  g of Pt-OEP in 5  ml of benzene was used to deposit the monolayer. Figure 1(c) shows a typical image of the Pt-OEP monolayer deposited from benzene. In this case, the Pt-OEP molecules form a slightly distorted hexagonal unit cell with an internal angle of about 57° and spacings of a~=1.2  nm and b~=0.8  nm. The contraction of the unit cell along the <i>b</i>-vector direction may be attributed to the effect of solvent on the adsorption geometry. This effect is not clearly understood and should be systematically studied. However, the difference in solvent does not explain why the Ni-OEP monolayer has a relatively larger lattice spacing. The line scan taken along the <i>a</i>-vector and <i>b</i>-vector directions shows that the height variations along these two directions are the same within the uncertainty of experimental data, suggesting that the molecule is still adsorbed flat on the surface. The smaller lattice spacing and higher packing density of Pt-OEP molecules are attributed to the weaker interaction between the Pt-OEP molecules and the HOPG surface, resulting in a close-packed monolayer formation.23

Figure 1.

In order to provide additional information about the interaction between Pt-OEP molecule and the surface, a 1-phenyloctane solution containing both 5-OIA and Pt-OEP (0.002  g of 5-OIA in 4  ml of solution A) has been prepared. From this solution, a complete monolayer containing only 5-OIA molecules readily forms on HOPG, driven by hydrogen bonding in the monolayer.10,24 The Pt-OEP molecules then adsorb on top of the 5-OIA monolayer. The topographic features of the 5-OIA SAM on bare HOPG have been previously studied.24,25 The majority of the surface is covered with a pure 5-OIA phase, in which two adjacent molecules in a lamella assemble with opposite faces in order to maximize the van der Waals interactions between the molecules. In addition, the head groups form three O–H[centered ellipsis]O bonds. Two of these bonds form between the nearest neighbors along the trough direction and the last one forms between the nearest molecules of the neighboring lamella. In this phase, the chain-to-trough angle is about 88°. (For further details, see Ref. 24.)

Figure 2(a) is a STM image of the Pt-OEP molecules adsorbed on the 5-OIA covered HOPG surface. The dark-striped areas visible through the vacancies in the Pt-OEP monolayer are the 5-OIA monolayer. Visible areas of the 5-OIA monolayer are aligned in different directions, suggesting that beneath the Pt-OEP monolayer, the 5-OIA monolayer has similar changes in the alignment, and there are boundaries separating these differently aligned regions. Some of these boundaries are even visible through the vacancies in the Pt-OEP monolayer in Fig. 2(a) (circled in the figure) and in Fig. 3. However, the STM images of the Pt-OEP monolayer do not show any indication of this phase separation in the Pt-OEP monolayer. Figure 2(b) is a high resolution STM image of the Pt-OEP monolayer formed above the 5-OIA monolayer. Similar to the Pt-OEP molecules adsorbed on the bare HOPG surface, the Pt-OEP molecules adsorbed on the 5-OIA covered HOPG surface form a monolayer with a hexagonal unit cell. The internal angle and the lattice spacing of the unit cell are still about 60° and 1.2  nm, respectively. The absence of any phase boundaries in the Pt-OEP monolayer and the unaffected unit cell structure of the Pt-OEP monolayer are additional evidences suggesting that the interactions between Pt-OEP molecules and the substrate are so weak that the lattice structure is determined and mainly kept stable by the interactions among the Pt-OEP molecules themselves.

Figure 2. Figure 3.

Sequentially measured STM images show a constant change in the morphology of the Pt-OEP monolayer adsorbed on the bare and the 5-OIA covered HOPG surfaces, indicating that the intermolecular interactions are not strong enough to create a fully rigid monolayer. As an example, Fig. 3 shows sequentially measured STM images of the Pt-OEP monolayer adsorbed on the 5-OIA covered HOPG surface. The circle and the square indicate regions where the Pt-OEP monolayer changes. The blurry region pointed out by the arrow in Fig. 3(a) shows a forming Pt-OEP island. In the following images, this island appears in a well defined shape. In order to investigate the effect of the tip on these changes, STM images were measured at different tunneling currents while keeping the tip bias constant. The STM images measured with higher tunneling currents clearly show a drastic change in surface morphology leading to complex and low resolution STM images (not shown).

In conclusion, by using STM at the solid-liquid interface, the adsorption of Pt-OEP molecules on bare and 5-OIA covered HOPG surfaces has been studied. On the bare HOPG surface, Pt-OEP molecules adsorbed from 1-phenyloctane form a hexagonal lattice with an internal angle of 60° and a lattice constant of a=b~=1.2  nm. These lattice spacings are significantly smaller than the results reported for the Ni-OEP molecules adsorbed from benzene and acetone solutions on the HOPG surface. Pt-OEP molecules adsorbed from benzene solution also form a slightly deformed hexagonal lattice with an internal angle of about 57° and lattice constants of a~=1.2  nm and b~=0.8  nm. The difference in the lattice structure indicates that the interaction between the Pt-OEP molecules with the HOPG surface is weak, allowing molecules to form a close packed monolayer. In further support of this claim, the adsorption of Pt-OEP molecules on the 5-OIA covered HOPG was examined. Although bare HOPG and 5-OIA covered HOPG surfaces have a quite different topography, on both surfaces, the Pt-OEP molecules form hexagonal lattices with the same internal angle and the same lattice constant. This shows that the underlying substrate has little effect on the adsorption geometry of the Pt-OEP molecules and that the Pt-OEP monolayer is primarily stabilized by intermolecular interactions within the layer.

This research was partially supported by the National Science Foundation, Division of Chemistry (CHE-0616457).

REFERENCES

Citation links [e.g., Phys. Rev. D 40, 2172 (1989)] go to online journal abstracts. Other links (see Reference Information) are available with your current login. Navigation of links may be more efficient using a second browser window.
  1. A. Aviram and M. A. Ratner, Chem. Phys. Lett. 29, 277 (1974). first citation in article
  2. D. H. Waldeck and D. N. Beratan, Science 261, 576 (1993). [ISI] [MEDLINE] first citation in article
  3. C. P. Collier, G. Mattersteig, E. W. Wong, Y. Luo, K. Beverly, J. Sampaio, F. M. Raymo, J. F. Stoddart, and J. R. Heath, Science 289, 1172 (2000). [MEDLINE] first citation in article
  4. A. H. Flood, J. F. Stoddart, D. W. Steuerman, and J. R. Heath, Science 306, 2055 (2004). [MEDLINE] first citation in article
  5. J. Kido, M. Kimura, and K. Nagai, Science 267, 1332 (1995). [Inspec] [ISI] [MEDLINE] first citation in article
  6. T. Malinski and Z. Tara, Nature (London) 358, 676 (1992). [ISI] [MEDLINE] first citation in article
  7. Y. Harima, H. Okazaki, Y. Kunugi, K. Yamashita, H. Ishii, and K. Seki, Appl. Phys. Lett. 69, 1059 (1996). [ISI] first citation in article
  8. T. A. Heimer, C. A. Bignozzi, and G. J. Meyer, J. Phys. Chem. 97, 11987 (1993). [Inspec] [ISI] first citation in article
  9. A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir: Blodgett to Self-Assembly (Academic, Boston, 1991). first citation in article
  10. F. Tao and S. L. Bernasek, J. Am. Chem. Soc. 127, 12750 (2005). [MEDLINE] first citation in article
  11. F. Tao, J. Goswami, and S. L. Bernasek, J. Phys. Chem. B 110, 19562 (2006). first citation in article
  12. B. C. Stipe, M. A. Rezaei, and W. Ho, Science 280, 1732 (1998). [MEDLINE] first citation in article
  13. A. S. Hallback, N. Oncel, J. Huskens, H. J. W. Zandvliet, and B. Poelsema, Nano Lett. 4, 2393 (2004). [ISI] first citation in article
  14. J. Loos, Adv. Mater. (Weinheim, Ger.) 17, 1821 (2005). [ISI] first citation in article
  15. B. Duong, R. Arechabaleta, and N. J. Tao, J. Electroanal. Chem. 447, 63 (1998). first citation in article
  16. C. H. M. Maree, S. J. Roosendaal, T. J. Savenije, R. E. I. Schropp, T. J. Schaafsma, and F. H. P. M. Habraken, J. Appl. Phys. 80, 3381 (1996). [ISI] first citation in article
  17. T. Tsuboi, Y. Wasai, and N. Nabatova-Gabain, Thin Solid Films 496, 674 (2006). [Inspec] first citation in article
  18. A. Bansal, W. Holzer, A. Penzkofer, and T. Tsuboi, Chem. Phys. 330, 118 (2006). [Inspec] first citation in article
  19. L. Scudiero, D. E. Barlow, and K. W. Hipps, J. Phys. Chem. 104, 11899 (2000). first citation in article
  20. A. Ogunrinde, K. W. Hipps, and L. Scudiero, Langmuir 22, 5697 (2006). [MEDLINE] first citation in article
  21. F. Tao and S. L. Bernasek, Langmuir 23, 3513 (2007). [MEDLINE] first citation in article
  22. C. J. Li, Q. D. Zeng, C. Wang, L. J. Wan, S. L. Xu, C. R. Wang, and C. L. Bai, J. Phys. Chem. B 107, 747 (2003). [Inspec] first citation in article
  23. T. G. Gopakumar, F. Muller, and M. Hietschold, J. Phys. Chem. 110, 6051 (2006). first citation in article
  24. F. Tao and S. L. Bernasek, Surf. Sci. 601, 2284 (2007). [Inspec] first citation in article
  25. P. Vanoppen, P. C. M. Grim, M. Rucker, S. De Feyter, G. Moessner, and S. Valiyaveettil, J. Phys. Chem. 100, 19636 (1996). [ISI] first citation in article

FIGURES

Full figure (78 kB)

Fig. 1. (Color online) (a) A 6.7×6.7  nm2 STM image of Pt-OEP adsorbed on the bare HOPG surface from 1-phenyloctane solution. The tip bias is 1  V and the tunneling current is 0.6  nA. (b) A line scan measured on Pt-OEP molecules shown in (a) (dotted blue line). The peak to peak distance is the lattice spacing between two molecules and it is about 1.2  nm. (c) A 3.7×3.7  nm2 STM image of Pt-OEP adsorbed on bare HOPG surface from benzene solution. (d) Two line scans measured along the <i>a</i>-vector and <i>b</i>-vector directions. The tip bias is 0.8  V and the tunneling current is 0.6  nA. Vectors <i>a</i>-vector and <i>b</i>-vector show the symmetry directions of the molecular layers. A model of the Pt-OEP molecule is overlaid on the STM images shown in (a) and (c). Since it is not possible to extract the spatial configuration of the ethyl groups, their position is arbitrarily assigned. First citation in article

Full figure (31 kB)

Fig. 2. (Color online) (a) A 150×150  nm2 STM image of Pt-OEP adsorbed on 5-OIA SAM. Circled regions show boundaries between differently aligned 5-OIA molecules. (b) A line scan taken over Pt-OEP molecules. (c) A 10×10  nm2 STM image of Pt-OEP adsorbed on 5-OIA SAM. For both STM images, the tip bias and tunneling current are 1  V and 0.6  nA, respectively. A model of the Pt-OEP molecule is inserted in the STM image shown in (c). Since it is not possible to extract the spatial configuration of the ethyl groups, their position is arbitrarily assigned. First citation in article

Full figure (34 kB)

Fig. 3. (Color online) (a)–(e) are sequentially measured 150×150  nm2 STM images of the same region. Blue circle and square indicate regions where a change in the monolayer is observed. (f) There is a lapse time of about half an hour between (e) and (f). During this time, various small scale images were measured around this region. First citation in article

FOOTNOTES

aAuthor to whom correspondence should be addressed. Electronic mail: sberna@princeton.edu.

Up: Issue Table of Contents
Go to: Previous Article | Next Article
Other formats: HTML (smaller files) | PDF ( kB)